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PCBD-Nov2014

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November 2014 • The PCB Design Magazine 29 COMPONENT SELECTION FOR EASIER DESIGN & MANuFACTuRE OF ELECTRONICS continues of the big reasons for the significant variation in component height within each class is that it is a significant challenge to make all of the solder balls sit in a common plane and there is thus an allowance for such non-uniformity within and between such components. Why use a Package at All? One might be tempted to leap ahead at this point and question the need for a package. There has been, for example, an increasing flow of papers and articles about embedding ICs into modules and PCBs. IPC and IEC have developed or are developing standards for embedded de- vice design and assembly, but they are proving rather confusing because both documents try to include every imaginable variation, some of which are not particularly practical. However, the underlying issue with using bare die in an electronic assembly is multifaceted and the de- signer must understand why they should be avoided. To begin, bare die are difficult to handle and place. They are delicate and can be easily dam- aged both physically and electrically by ESD. Second, it is very difficult to assure die quality and reliability because burn in of discrete die (or even in wafer form) is difficult and even when done, it is expensive. Moreover, most chip foundries do not like shipping bare die because of these concerns and others related to business as yield can be determined. A third reason why bare die are not a good choice is that they have no standards. Every IC chip is unique and rarely are chips provided with a pad layout that has any kind of standard grid pitch. (There are a few article Figure 1: a comparison on JeDeC defined mechanical heights for electronic components. The wide range of thickness is due to a combination of legacy mentality which was not as concerned with height and the fact that solder termination height can vary significantly within and between parts. Note also that roughly half of the height of a component is due to the solder used for interconnection. a com- ponent designed for saFe products can, in many cases, be either a u or X designation because of the elimination of solder.

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